Answer: Well-differentiated mucinous adenocarcinoma of lung

Histology: Sections show fragments of well differentiated mucinous adenocarcinoma with a predominantly lepidic growth pattern. Focal cribriform and papillary structures, as wells as, associated mild interstitial fibrosis is present. The majority of tumor cells have basally located nuclei indented by intracellular mucin vacuoles, but focally enlarged nuclei with moderate pleomorphism located in the center of the cells are also present. Immunostains demonstrated that the tumor cells are positive for CK7, BER-EP4, M-CEA and Napsin-A, focally positive for CK20, TTF-1, CK5/6 and p63, while negative for MUC-2 and CDX-2.

Discussion: The 2011 IASLC/ATS/ERS classification of lung adenocarcinoma recommends that in small biopsies the diagnosis of adenocarcinoma in situ (formerly called bronchoalveolar carcinoma – BAC) should not be made since the entire tumor is not available for evaluation. The preferred nomenclature is well differentiated adenocarcinoma with lepidic growth pattern and in the case of purely mucinous tumors, as in this case, well differentiated mucinous adenocarcinoma. It is also emphasized that the diagnosis of mucinous adenocarcinoma in situ (mucinous BAC) is rare and should be restricted to cases that have a uniform monotonous cytology with basally located indented nuclei and a purely lepidic growth pattern. Any tumor showing other morphologies and tumors greater than 3 cm should be considered invasive and diagnosed as mucinous adenocarcinoma. Primary mucinous adenocarcinoma of the lung can be very difficult or impossible to distinguish from metastatic mucinous tumors both by histology and immuno phenotype. A minority of primary mucinous adenocarcinoma of the lung (10-15%) can be TTF-1 positive, while even fewer cases may show positivity for Napsin-A. CK7 versus CK20 positivity also favors lung origin if the differential diagnosis is between lung and lower gastrointestinal tract origin.